The focus of this grant is randomized clinical trials for hematologic disorders, which require a multi-center approach in the NHLBI Transfusion Medicine/Hemostasis Clinical Research Network. Three Harvard teaching hospitals form a consortium for this Core Clinical Center application. Key linkages among the institutions are in place, including the Joint Program in Transfusion Medicine, and the Boston Hemophilia Center. Adult and pediatric hematology and transfusion medicine services are represented, as well as collaboration with the high-risk obstetrics services at our institutions. The first proposed study has a pediatric focus and two-year time frame. The aim of this randomized phase II trial is to assess the efficacy of rituximab (anti-CD20 monoclonal antibody) vs. azathioprine, in children and adolescents with severe or refractor chronic idiopathic thrombocytopenic purpura. The primary efficacy outcome will be platelet counts at study day 90. Secondary outcomes include bleeding score trend, platelet counts at one year, side effects of medication, and requirement for 'salvage' regimens during either course of therapy. Our proposed long-term study will focus on randomized treatment strategies in thrombotic thrombocytopenic purpura (TTP). The aim is to determine whether rituximab therapy in addition to prednisone and plasmapheresis will facilitate remission induction, compared to standard therapy of plasmapheresis/ prednisone alone. Primary efficacy outcomes include the fraction of patients alive with no more than 9 plasma exchange procedures at 30 days from diagnosis (early responders) and the fraction of patients alive and relapse-free at 24 months. Secondary endpoints will include the death rate, the fraction of patients in remission at 30 days, the time to first remission in each treatment group, the number of plasma exchange procedures per patients, the number of relapses per group, and the time to remission and relapse rate, in each group, stratified for the presence of absence of VWV metalloprotease inhibitors and quantification of VW protease activity. Third, we propose a multicenter consortium for a phase III randomized study comparing two different dosage regimens of intravenous gamma globulin during pregnancies at risk for neonatal alloimmune thrombocytopenia. A repository for sera, plasma, and DNA from patients in each of the transfusion network studies is proposed, to facilitate further biological studies.